The present invention relates to a laminate film with excellent durability in use under pressure and heating, and also a fixing film for use in a heating apparatus for heating a material to be heated, or in an image forming apparatus, such as an electrophotographic apparatus or an electrostatic recording apparatus, equipped with such a heating apparatus as an image heating apparatus for heat fixing a yet-unfixed image carried on a recording material. The present invention further relates to such an image heating apparatus.
For convenience of explanation, an image heating apparatus for heat-fixing a toner image onto an image forming apparatus, such as a copying machine or a printer, is described as an example.
Hitherto, a heating roller-type apparatus has been widely used as an image heating apparatus for heat-fixing a yet-unfixed image formed by transfer or directly on a recording material (such as a transfer sheet, an electrofax sheet, an electrostatic recording sheet, an OHP sheet, a printing paper or a format paper) by an appropriate image forming process unit according to an electrophotographic process, an electrostatic recording process.
Such a roller-type heating apparatus basically comprises a fixing roller (heating roller) and a pressure roller constituting a pair of mutually abutting rollers. The rollers are rotated while forming a fixing nip (heating nip) at a mutually abutted portion therebetween, where a recording material carrying a yet-unfixed toner image is introduced and the yet-unfixed toner image is fixed onto the surface of the recording material under the action of heat from the fixing roller and a pressure at the fixing nip. The roller-type heating apparatus is accompanied with a problem that the fixing roller has a large heat capacity, so that a substantial time is required until the roller reaches a prescribed fixing temperature, and in order to acquire a quick start-up performance, the fixing roller has to be kept heated at a certain controlled temperature.
For solving the above problem, a film heating-type fixing apparatus has been proposed, e.g., by Japanese Laid-Open Patent Application (JP-A) 63-313182, JP-A 2-157878, JP-A 4-44075 and JP-A 4-204980.
More specifically, in the film heating-type fixing apparatus, a ceramic heater as a heating member and a pressure roller as a pressing member are caused to nip a heat-resistant film (fixing film) to form a nip (portion), where a recording material carrying a yet-unfixed toner is introduced between the film and the pressure roller and conveyed while being nipped together with the film to transfer the heat from the ceramic heater to the recording material via the film and heat-fix the yet-unfixed toner image onto the recording material surface under a pressure at the nip.
As a characteristic of the film heating-type fixing apparatus, the ceramic heater and the film can be composed of low-heat capacity members to provide an on-demand type device, thus allowing an image forming apparatus wherein the ceramic heater as the heat source is energized to be heated to a prescribed fixing temperature only at the time of image formation, so that the waiting time from the turning-on of the power supply of the image forming apparatus until reaching the image-forming allowable state is short (quick start characteristic) and the power consumption during the stand-by period is remarkably smaller (power economization).
As the film used in the film heating-type apparatus, there has been used a film comprising a heat-resistant resin as represented by polyimide resin excellent in heat-resistance and strength. However, in the case of providing a fixing apparatus adapted to high-speed and high durability use, a resin film alone is insufficient in strength and is therefore desirably backed with a base layer of a metal excellent in strength, such as SUS (stainless steel), nickel, aluminum or copper.
Further, JP-A 7-114276 has proposed a heating apparatus including an electroconductive member constituting a film per se or disposed in proximity with the film, where an eddy current is generated to cause a Joule heat as a source of heating (electromagnetic induction heating system). According to the electromagnetic induction heating system, the heating region can be shifted closer to the material to be heated, so the efficiency of consumed energy can be improved.
The heating apparatus of the electromagnetic induction heating scheme, a cylindrical or endless film as a rotating member is driven in rotation, while being sandwiched under pressure between a film guide and a pressure roller, by following the rotation drive of the pressure roller (i.e., the pressure roller-drive scheme) or driven by itself, while being stretched under tension about a drive roller and a tension roller, so that the pressure roller is driven in rotation by following the drive of the film.
Such a film used in the electromagnetic induction heating generally comprises as a base layer a metal film functioning as an electromagnetic induction heating member and particularly comprising a magnetic material, such as iron, nickel, ferromagnetic SUS (stainless steel) or nickel cobalt alloy.
As a problem in the film heating scheme or the electromagnetic induction heating scheme in the case where the fixing film has a laminate structure including a layer of a metal, such as nickel, the fixing film is liable to show a large frictional resistance with a film guide member at the nip. In a case where a lubricant, such as a heat resistant grease, is placed between the film inner surface and the film guide, a portion of the metal film and the film guide are worn due to continual use and mixed with the grease to deprive the grease of the lubricating effect, thus resulting in an increase in rotation torque.
If a laminate film of a metal layer and a polyimide resin layer is used so as to have the polyimide resin layer surface contacting the film guide, the increase of rotation torque can be prevented, but in this case, the laminate film is caused to have a durability which is lower than that of the uncoated metal layer, by promoting the breakage of the metal layer, so that long-term continuous use under heating and pressure becomes difficult.
A generic object of the present invention is to provide a laminate film having solved the above-mentioned problem.
Other objects of the present invention are to provide a fixing film showing excellent durability in use under heating and pressure and to provide an image heating apparatus.
According to the present invention, there is provided a laminate film, comprising a metal layer and a polyimide resin layer, wherein the polyimide resin layer has an imidation percentage of 70-93%.
The present invention further provides a fixing film, comprising a metal layer and a polyimide resin layer, wherein the polyimide resin layer has an imidation percentage of 70-93%.
The present invention also provides an image heating apparatus, comprising: a fixing film comprising a metal layer and a polyimide resin layer, and a pair of pressing members pressed against each other via the fixing film, so that the polyimide resin layer of the fixing film is moved in contact with and relative to one of the pressing members and the fixing film supplies, heat to an image on a recording material, wherein the polyimide resin layer of the fixing film has an imidation percentage of 70-93%.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.